“There is no worse calamity for science and for scholarship than those intruders who are foreign to them. They are ignorant and yet think they know; they ruin everything whilst convinced they are fixing all.”

volume I, "Introduction", page 3 http://darwin-online.org.uk/content/frameset?pageseq=16&itemID=F937.1&viewtype=image
Source: The Descent of Man (1871)
Context: It has often and confidently been asserted, that man's origin can never be known: but ignorance more frequently begets confidence than does knowledge: it is those who know little, and not those who know much, who so positively assert that this or that problem will never be solved by science.

Lecture IX : On the Conduct of the Understanding
Elementary Sketches of Moral Philosophy (1849)

Conditions of Progress in Democratic Government (1909).
Context: No greater mistake can be made than to think that our institutions are fixed or may not be changed for the worse. … Increasing prosperity tends to breed indifference and to corrupt moral soundness. Glaring inequalities in condition create discontent and strain the democratic relation. The vicious are the willing, and the ignorant are unconscious instruments of political artifice. Selfishness and demagoguery take advantage of liberty. The selfish hand constantly seeks to control government, and every increase of governmental power, even to meet just needs, furnishes opportunity for abuse and stimulates the effort to bend it to improper uses... The peril of this Nation is not in any foreign foe! We, the people, are its power, its peril, and its hope!

Dear Me (1977)

As quoted in The Mammoth Book of Zingers, Quips, and One-Liners (2004) edited by Geoff Tibballs, p. 299
General sources

Source: Lullaby (2002), Chapter 39

Reported in Josiah Hotchkiss Gilbert, Dictionary of Burning Words of Brilliant Writers (1895), p. 36.

As quoted in Louis Pasteur, Free Lance of Science (1960) by René Jules Dubos, Ch. 3 "Pasteur in Action"